These tanks, in which ballast water is carried in ships, have no physical connection to fuel or cargo tanks. As a result, water in these tanks is not contaminated, as is ballast water carried in fuel or cargo...

see Segregated ballast...

A ballast tank is a compartment within a boat or ship, that holds water. A vessel may have a single ballast tank near its center or multiple ballast tanks typically on either side. A large vessel typically will have several ballast tanks including...